Parts assembly using the conventional grippers of the type which have facing gripping surfaces between which the part to be grasped is located cannot be conveniently used in certain assembly tasks where the parts to be assembled are to be stacked in a narrow casting. It will be appreciated that the fingers holding the part or component to be fully inserted in a slightly larger diameter opening will contact the casting or housing interfering with the insertion. Prior devices have been disclosed for overcoming this problem such as grasping means for picking up articles having openings therein, as shown in U.S. Pat. No. 4,273,505, which discloses an end effector for a robot, which includes an inflatable balloon member carried on the end of a tubular member which has a hollow center or conduit through which a suitable pressurized fluid is supplied. The tubular member is inserted within an opening in the part to be grasped and is inflated to provide the grasping. Likewise, U.S. Pat. No. 4,173,368 discloses apparatus for frictionally gripping the interior surface of a container during handling. The patent teaches applying air pressure to a cup causing a piston located therein to be driven downwardly causing straps to flex radially outwardly so as to frictionally grip the interior surface of a container or part to be grasped. Both patents require pneumatic means for operating the relatively complicated end effector in addition to the robot positioning means for locating the end effector in proximity to the interior surfaces to be gripped.
One solution to the problem of providing complicated adjustable size gripping means is to provide end effectors or tools which are readily changeable. That is, when a part of one size is to be inserted, a tool is picked up and automatically connected to the robot to perform the given task. A different size part will require a different size tool and, accordingly, the tool is automatically changed to fit the new task. An automatic tool changing mechanism is disclosed in U.S. application Ser. No. 565,444 filed Mar. 16, 1984.